I made a top for my sons 20 gal long tank, by cutting a piece of 3/4 inch plywood to the dimentions of the top
( for example 56 X 26 inches ) then use 1 X 3 ( or similiar ) to trim the plwood, so that when you place the lid on the tank, the plywood holds it place. You can even attach the trim onto the plywood, so that you have a ridge or rim around the top of your top.
I cut a door through the top and attached hinges and a throw bolt style latch, but you could cut holes through the top for a vent, heat, or whatever.
Try to make your cuts tight close, so that the trim will fit snuggly around the top of your tank, not that hard to do and you don't need to make any fancy 45 deg. cuts for your corners, butt joints will work just fine.
Then you can finish it, either by painting it with a acrylic paint ( or ) stain and seal.
Looks a whole lot better than a store bought screen cover.